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Unformatted text preview: high moist high temp low moist. low temp q v R n low moist intermed. temp. high press anti cyclone 90 N 90 S 90 60 30 N 0 30 60 90 S AOM 4932 - Atmospheric Water and Precipitation Distribution of atmospheric moisture in space and time. In general: 1 - water vapor by volume (% of total) decreases with elevation (most within 5 km, > 8 km approximately none) 2 - specific humidity = v / moist increases and decreases seasonally with temperature (warm air can hold more water vapor) 3 - specific humidity is highest in the tropics and lowest in the poles (for same reason as 2) 4 - relative humidity = v / moist shows peaks both in tropics and in poles -- minimum at high pressure regions (30 - 40 ) Knowledge of vertical and horizontal spatial distribution of moisture allows computation of potential precipitable water in an area. However, for precipitation to occur atmospheric moisture must condense. In the atmosphere this typically occurs when air temperature is lowered when the air mass is forced to rise. Formation of Precipitation Requires: 1 - Cooling of air to dew point temperature (requires a lifting mechanism) 2 - Condensation of water vapor onto nuclei (dust, ions) to form droplets 3 - growth of droplets so that a) terminal velocity updraft velocity b) sufficient mass of liquid to survive evaporation on way down 4 - Importation of water vapor into cloud to replace precipitation and sustain process 1 - Lifting Mechanisms Three meteorological situations which lead to vertical uplift of air masses: a) uplift due to convergence Nonfrontal convergence of air masses with equal temperature to a low pressure point (i.e. at ITCZ due to convergence of NE/SE tradewinds). Generates moderate rainfall over long duration. Frontal convergence of air masses of different air temperature. Produces cold fronts/warm fronts. warm front- Occurs when warm air impinges on cold. Two air masses do not mix. Warm moist air is less dense, rises over cold air at relatively gentle slope. Warming occurs gradually resulting in more moderate storms which last longer. See high clouds first cold front- Cold air impinges on warm air. Again do not mix but cold air moves under warm forcing it upward. Get a steeper sloped interface. Rapid cooling, stronger storms of shorter duration. See low clouds first b) uplift due to convection Convective cells are initiated by heating of lower air mass by ground surface. Cause instability of air column because of density differentials ( T, ). Lower air density rises and cools and condenses (releases heat...
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- Fall '11